I get that. I should have used the word warning instead of error but is there a way to have this warning not appear?

It's a warning that there are some null ef.end_dt values and that those can't be 'maxed' right? I thought I'd be handling the warning by using the isnull function which creates a dummy date in the case where there are null values. Is there a way that I can run this that the warning won't happen?